Reduced mask count process for manufacture of mosgated device

ABSTRACT

A process for forming a power MOSFET enables the connection a metal gate electrode to the conductive polysilicon gates in the active area without an additional mask step. In the process, a groove is formed in the field oxide during the active area mask step. Conductive polysilicon is then formed over the active area and into the groove. At least one window is formed over the groove along with the mask window for forming the channel and source implant windows, and the polysilicon is etched to the silicon surface in the active area, but a strip is left in the groove. This strip is contacted by gate metal during metal deposition. Thus, gate metal is connected to the polysilicon without an added mask step.

RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 60/277,637, filed Mar. 21, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to semiconductor device processing and more specifically relates to a novel reduced mask count process for the manufacture of a MOSgated semiconductor device and a novel resulting device structure.

BACKGROUND OF THE INVENTION

[0003] MOSgated devices, such as power MOSFETs and IGBTs are very well known, and have a gate electrode, usually made of conductive polysilicon, which must be electrically insulated from the source electrode (or emitter electrode) and yet must be electrically connected to an external metallic gate conductor or gate pad, usually of aluminum. In the self aligned contact power MOSFET, contact to the source is made in the active area, using spacer technology. However, the gate metal-to-gate polysilicon contact at the gate pad has required the use of a separate photo mask step.

[0004] It is desirable to reduce the number of photomask steps needed in a manufacturing process to reduce cost and to increase yield of devices from a wafer.

[0005] The present invention provides a novel process which eliminates the need for a separate mask for the gate polysilicon to gate metal connection in a manufacturing process.

SUMMARY OF THE INVENTION

[0006] In accordance with the invention, a groove is formed in the P⁺ field oxide during the active mask step in which the field oxide layer is otherwise etched to open the active area. Thus, no subsequent added mask is needed to form this groove. Thereafter, and during polysilicon deposition, the groove is filled as the polysilicon is deposited atop the field oxide.

[0007] During the polysilicon mask etch step additional windows are opened in the polysilicon layer above the groove in the oxide and in the area where the polysilicon to metal contact is to take place. Again no added mask is used and the necessary polysilicon mask is simply modified for this added window.

[0008] Next, the conventional polyoxide and polysilicon etch and insulation spacer steps are carried out, and the polysilicon remaining in the bottom of the groove is exposed.

[0009] The conductive metal contact is then deposited atop the polyoxide and it fills the window(s) previously formed and contacts the polysilicon in the groove, which is connected to the body of the polysilicon layer. Thus, the contact is made without the need for an extra mask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a portion of a semiconductor die, in cross-section, with field oxide which has been patterned to expose the active area surface and a groove which can receive a contact metal contact to the polysilicon subsequently formed in the active area.

[0011]FIG. 2 shows the cross-section of FIG. 1 after the deposition of polysilicon.

[0012]FIG. 3 shows the structure of FIG. 2 after the formation of a polyoxide layer atop the polysilicon and a photoresist layer (patterned) atop the polyoxide layer.

[0013]FIG. 3A is a schematic isometric view of a portion of FIG. 3 illustrating one window patterned in the photoresist atop the groove in the filed oxide.

[0014]FIG. 3B is a cross section of FIG. 3A taken across section line 3 b-3 b in FIG. 3A.

[0015]FIG. 3C is a cross-section of FIG. 3A taken across section line 3 c-3 c in FIG. 3A.

[0016]FIG. 4 shows the structure of FIG. 3 after the etch of the polyoxide and polysilicon areas exposed by windows in the photo resist; the formation of insulation spacers over the exposed edges of the polysilicon layer; and the deposition of a contact metal.

[0017]FIG. 4A shows the structure of FIG. 3B after the process steps of FIG. 4.

[0018]FIG. 4B shows the structure of FIG. 3C after the process steps of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Referring first to FIG. 1, there is shown a silicon die 10 which has an N⁻ epitaxial layer grown atop an N⁺ substrate (not shown) of conventional structure. The invention applies equally to devices with a P type substrate. A field oxide layer 11 is conventionally deposited or otherwise formed atop substrate 10, and, in an active area mask step, the large active area 12 has field oxide removed therefrom to enable the subsequent formation of the junction pattern in the active area. In accordance with the invention, a further window or windows in the active area mask permits the formation of one or more grooves 12, which, as will be later seen, will permit the gate metal to contact the polysilicon gate in the active area without an added mask step.

[0020] As next shown in FIG. 2, a gate oxide layer 13 is thermally grown atop the exposed active silicon area which was exposed in FIG. 1, and a conductive polysilicon layer 14 is formed atop gate oxide 13, as well as over the groove 12 and remaining field oxide segments 11.

[0021] Thereafter, and as shown in FIG. 3, 3A, 3B and 3C a polyoxide layer 15 is conventionally formed atop polysilicon layer 14. A photoresist layer 16 is then formed atop polyoxide layer 15 and is a patterned in a mask step to open windows 17, 18, 19 and 20 and other similar windows. Mask windows 18, 19 and 20 are conventional implant windows used for the formation of channel diffusion regions 21 and source diffusion region 22 for a vertical conduction MOSFET. (A drain contact will be formed on the bottom of die 10.) These diffusions 21 and 22 may be cellular or in stripe form and can have any desired size, depth and topology. Mask window 17 (FIGS. 3, 3A, 3B and 3C is also formed at this same mask step, disposed over an aligned with groove 12. Any number of windows 17 can be used.

[0022] An etch process is then used to remove the portions of polyoxide layer 15, polysilicon layer 14 and gate oxide layer 13 which are exposed by windows 17 to 20. Note that a bottom line portion 50 of polysilicon layer 14 remains at the bottom of groove 12 after this etch process (FIGS. 4, 4A and 4B). Note also that no added mask step is needed to form polysilicon line 23, which is integrally connected to the polysilicon member (gate electrodes) 14.

[0023] Thereafter, a conductive contact layer 31, which may be aluminum is deposited as by sputtering, to make contact with polysilicon line 50 and the source and channel regions 22 and 21. A contact mask step then permits the removal of metal in gap 33 (FIG. 4) to separate gate metal and source metal as shown in FIG. 4. As shown in FIGS. 4, 4A and 4B, the gate metal segment is directly connected to polysilicon strip 23, and thus to polysilicon gates 14 over the invertible channel regions on the active area. Significantly, no added mask is needed with the novel process.

[0024] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein. 

What is claimed is:
 1. A reduced mask count process for the manufacture of a MOSgated device comprising the steps of: forming a field oxide on the surface of a silicon substrate; opening an active area in said field oxide, and, at the same time, opening at least one groove in a portion of the remaining field oxide in a common mask step; forming a layer of polysilicon over the upper surface of said active gate oxide and into said at least one groove; opening a plurality of spaced windows over the active area and at least one window over said at least one groove in a further common mask step and etching away the polysilicon exposed by said windows in the active area and only a portion of the polysilicon in said at least one groove, leaving a strip of polysilicon in the bottom of said groove which is connected to the polysilicon remaining in the active area; and forming a metal gate contact to said strip of polysilicon in the bottom of said groove.
 2. The process of claim 1, which includes the further step of forming a polyoxide layer over the surface of said polysilicon prior to the opening of said plurality of windows.
 3. The process of claim 1, which includes the further step of implanting and diffusing channel and source impurities through said plurality windows and into the silicon exposed by the etch of said polysilicon in said active region.
 4. The process of claim 2, which includes the further step of implanting and diffusing channel and source impurities through said plurality windows and into the silicon exposed by the etch of said polysilicon in said active region.
 5. The process of connecting a metal gate to a conductive polysilicon gate electrode in a MOSgated device; said process comprising the steps of forming a groove in an oxide layer which is atop a portion of a silicon surface; depositing a polysilicon gate electrode layer over said oxide and into said groove and over the portion of said silicon surface which is removed from said oxide layer; and etching openings through selected portions of said polysilicon surface and simultaneously etching away only a portion of the polysilicon disposed in said groove and leaving a strip of said polysilicon layer in the bottom of said groove which is connected to the polysilicon remaining in said active layer; and thereafter contacting said strip with a metal gate contact.
 6. The process of claim 5, wherein said metal gate contact is connected to said polysilicon layer without an extra mask step.
 7. A MOSgated semiconductor device; said MOSgated device having an active silicon area which contains an extending polysilicon gate layer; said active silicon area having a region which has a field oxide layer; said field oxide layer having a groove therein; a shallow strip of polysilicon disposed in the bottom of said groove and connected to said extending polysilicon gate layer; and a metal gate contact disposed in said groove and in contact with said strip of polysilicon. 